It seems like you have to create a unique pseudorandom alphanumeric string on your own (it doesn't matter how you create it.) Then pass it down to the link:
http://www.clickbank.net/sell.cgi?link=seller/1/test&seed=41FLZ9FJJ
"link" parameter has to have your own store code. The unique string has to be passed with the parameter name "seed" so if your string is "4s3s32" then you simply add "seed=4s3s32" in the URL.
After the purchase is made, the clickbank.net will run some script and return you a variable called "cbpop". You use the provided function to find out if the transaction is successfully made. Using the provided function with the right parameter values, you get 1 if it is successful and 0 if not.
You just redirect your customer to a page if the function returns 1, or redirect to a error page saying that the transaction doesn't go through so they have to try it again.
